FIELD OBSERVATIONS OF PRISTINE BLOCK-FLOW AND ASH-FLOW DEPOSITS EMPLACED APRIL 16-17, 1991 AT VOLCAN-DE-COLIMA, MEXICO

For the first time since the effusive activity of 1981-1982, Volcan de Colima extruded scoriaceous andesitic lava on March 1 1991. This followed a period of increasing seismicity, summit dome deformation, and fumarolic activity that began in February of 1991. Growth of the new lava dome proceeded until April 16 at 16:00 hrs (local time, GMT -6 hours), when a series of gravitational failures of the upper edifice and the new lava dome-flow triggered the emplacement of successive small-volume, non-glowing, block- and ash-flows to a distance of 4 km within the upper reaches of Barranca El Cordoban, south-southwest of the present summit. Aerial photos taken at the time of the event clearly show the lack of explosions at the summit dome. Efficient elutriation of the finest silt and clay-sized fraction from the main body of the avalanching material produced large reddish, yellowish, and grayish ash clouds which rose to an altitude of 1. 5-2.5 km for several hours and were carried southeast to northeast by predominant winds. Vegetation in the path of the debris was totally stripped, sand-blasted, and bent in the sense of flowage but was not burnt. No fatalities were reported. We present a summary of our field observations of the pristine deposits which we studied prior the rainy season, on April 20-27 between 2,200 and 2,500 m elevation.